This invention relates generally to processing of a stream of air prior to its separation into components, and more particularly concerns efficient drying and cooling of such air stream.
Air separation units (ASU) separate air into its constituent parts, nitrogen, oxygen and Argon. This is performed by distillation at low temperatures (−300 DegF.). Preliminary to cooling the air feed stock to the liquefaction point, it is necessary to remove the minor amounts of water and carbon dioxide present in air, prior to the introduction of air to the heat exchangers where the air is exposed to freezing temperatures. In modern plants this is done by a two step process. First the compressed air is cooled to about 5 DegC. (41 DegF.), where most of the water is removed by condensation and separation. Next the cooled air is passed through absorbent beds containing a suitable absorbent such as a molecular sieve, where the last traces of moisture and the carbon dioxide are removed. The reduced air temperature is necessary to provide the absorbent function with a high degree of affinity for carbon dioxide. The beds are regenerated periodically by either de-pressurization (Pressure Swing Absorption) or more commonly heating (Temperature Swing Absorption).
There is need for improvements in such processes, which typically employ mechanical refrigerators running with either Freon or ammonia. The evaporator operates at temperature close to freezing to prevent the water in the compressed air from freezing on the tube surfaces. The heat absorbed in cooling the air and condensing the water combined with the power used in the compressor is rejected to either air (ambient) or a cooling water circuit.